worked alone or in pairs as to not draw attention to our activities.
Slipped into first person.
send teams into the each of the entrances to the sewers t
Extra "the".
began. "These things have stalked the people in this city for months without our knowledge, we will not allow their deaths to go unavenged".
This bit is weird, I initially thought he wouldn't let the monster's deaths go unavenged. Should probably finish with them stalking the city for months without their knowledge, as it goes well with the initial "be wary", the "these things are dangerous/sneaky" goes unsaid but clearly heard.
I would give for advice, besides what's already said:
1) It's sometimes repetitive. You often say things like "he was serious about this, as he had brought in help to deal with this". Instead of saying "when he entered the room, he saw that", you could say "entering the room, he saw that" or "he could see the Maester was serious about this, having brought in outside help [to deal with it]."
2) The dialogue is somewhat stilted. Build up the scene some more.
3) Somewhat related to 2, but you could use some extra verbs and adverbs here and there. "A mighty thrust", "a hefty blow", "fast as a viper, he strikes", "darkness loomed beyond the pooled light of their lanterns", "he furiously stomped on the chittering masses", "his gaze sharpened as he leaned forward, drawing the other's attentions to his next words."
In general, describe and qualify how and what people are doing, as to better paint the scene, you get me? Flesh things out, otherwise it feels rushed and disconnected.
Instead of going blow-for-blow in a somewhat repetitive fashion, try to condense things. Roll the combat while jotting down the rolls to help you keep track of what happened, and then build a narrative around the results. For example, while Ser Richard can land four attacks in a round, he often decapitates or disembowels foes "in a single strike", even as sometimes he "rains blows down upon the demon".
It's very, very hard to pull a blow-by-blow battle scene. If you want to do it, I recommend reading Cornwell's Saxon Chronicles, as you all but feel the mud on your boots and the blood on your face as you struggle mightily upon the shield wall, swords flashing and axes biting, the screams of the dying only drowned by the dim of battle.